Re: Add a loop versioning pass

[David Malcolm <dmalcolm@redhat.com>]

On Wed, 2018-10-24 at 14:05 +0100, Richard Sandiford wrote:
> This patch adds a pass that versions loops with variable index
> strides
> for the case in which the stride is 1.  E.g.:
> 
>     for (int i = 0; i < n; ++i)
>       x[i * stride] = ...;
> 
> becomes:
> 
>     if (stepx == 1)
>       for (int i = 0; i < n; ++i)
>         x[i] = ...;
>     else
>       for (int i = 0; i < n; ++i)
>         x[i * stride] = ...;
> 
> This is useful for both vector code and scalar code, and in some
> cases
> can enable further optimisations like loop interchange or pattern
> recognition.
> 
> The pass gives a 7.6% improvement on Cortex-A72 for 554.roms_r at -O3
> and a 2.4% improvement for 465.tonto.  I haven't found any SPEC tests
> that regress.
> 
> Sizewise, there's a 10% increase in .text for both 554.roms_r and
> 465.tonto.  That's obviously a lot, but in tonto's case it's because
> the whole program is written using assumed-shape arrays and pointers,
> so a large number of functions really do benefit from versioning.
> roms likewise makes heavy use of assumed-shape arrays, and that
> improvement in performance IMO justifies the code growth.
> 
> The next biggest .text increase is 4.5% for 548.exchange2_r.  I did
> see
> a small (0.4%) speed improvement there, but although both 3-iteration 
> runs
> produced stable results, that might still be noise.  There was a
> slightly
> larger (non-noise) improvement for a 256-bit SVE model.
> 
> 481.wrf and 521.wrf_r .text grew by 2.8% and 2.5% respectively, but
> without any noticeable improvement in performance.  No other test
> grew
> by more than 2%.
> 
> Although the main SPEC beneficiaries are all Fortran tests, the
> benchmarks we use for SVE also include some C and C++ tests that
> benefit.
> 
> Using -frepack-arrays gives the same benefits in many Fortran cases.
> The problem is that using that option inappropriately can force a
> full
> array copy for arguments that the function only reads once, and so it
> isn't really something we can turn on by default.  The new pass is
> supposed to give most of the benefits of -frepack-arrays without
> the risk of unnecessary repacking.
> 
> The patch therefore enables the pass by default at -O3.
> 
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?
> 
> Richard
> 

[...snip...]

> +/* Run the pass and return a set of TODO_* flags.  */
> +
> +unsigned int
> +loop_versioning::run ()
> +{
> +  gcc_assert (scev_initialized_p ());
> +
> +  if (!analyze_blocks ()
> +      || !prune_conditions ()
> +      || !make_versioning_decisions ()
> +      || !implement_versioning_decisions ())
> +    return 0;
> +
> +  return TODO_update_ssa;
> +}
> +
> +/* Loop versioningting pass.  */

(typo)

> +
> +namespace {

Could the whole file be within this anonymous namespace, rather than
just the opt_pass subclass?  (hiding class loop_versioning, so that the
optimizer knows that the only thing visible outside the TU is
make_pass_loop_versioning).  This can be a pain to debug, but you can
always comment out the anon namespace locally when debugging.

> +
> +const pass_data pass_data_loop_versioning =
> +{
> +  GIMPLE_PASS, /* type */
> +  "lversion", /* name */
> +  OPTGROUP_LOOP, /* optinfo_flags */
> +  TV_LOOP_VERSIONING, /* tv_id */
> +  PROP_cfg, /* properties_required */
> +  0, /* properties_provided */
> +  0, /* properties_destroyed */
> +  0, /* todo_flags_start */
> +  0, /* todo_flags_finish */
> +};
[...snip...]

> +
> +} // anon namespace
> +
> +gimple_opt_pass *
> +make_pass_loop_versioning (gcc::context *ctxt)
> +{
> +  return new pass_loop_versioning (ctxt);
> +}

[...snip...]